I run sundwon sa-12's in my boat and really like them for the price. Brand new you can get them for $180. They can take a 1000 watt rms all day long. Just be sure to run them in a ported or 4th order enclosure they don't do as good in sealed. There not a marine sub but I have had one mine in for 4 years and it still going.

My system will have 6 wetsounds xs-650's on a wetsounds ht6 amp, a pair of rev 10's powered by a syn 4. Does anyone have any suggestions for a subwoofer that would go well with this system?

You are going to want something along the lines of a powerful 12-inch woofer to keep pace with and be in balance with a system of your description. Although two of the right 10-inch subs will have just as much output if not a tad more.
Then again, balance may not be what you are looking for.
The missing part of the equation is the boat. That is where you really need to begin in your woofer and enclosure selection and matching process. The sub location, orientation, loading method, and more will help you pick the optimum enclosure displacement which in turn will heavily dictate the right sub driver. This is the way to proceed and place the horse before the cart. I wouldn't want to buy a woofer and then retro that into a box and retro that into a boat. Too many compromises made along the way going that route.

You are going to want something along the lines of a powerful 12-inch woofer to keep pace with and be in balance with a system of your description. Although two of the right 10-inch subs will have just as much output if not a tad more.
Then again, balance may not be what you are looking for.
The missing part of the equation is the boat. That is where you really need to begin in your woofer and enclosure selection and matching process. The sub location, orientation, loading method, and more will help you pick the optimum enclosure displacement which in turn will heavily dictate the right sub driver. This is the way to proceed and place the horse before the cart. I wouldn't want to buy a woofer and then retro that into a box and retro that into a boat. Too many compromises made along the way going that route.

David

Thank you David,
I am picking up my boat next week. Maybe a good start would be for me to take some pictures of the places I can install a subwoofer and then go from there. You're right, I wouldn't want to spend good money on a subwoofer and not have it perform like it should because of a poor fit.